This invention relates to the field of charged particle generation for high energy electronic apparatus.
Pulsed electron beam devices which operate at high power levels and high voltages (powers in the gigawatts and voltages above 100 kilovolts, for example), need electron sourcing cathodes capable of operating at high current densities (densities of 100 amperes per square centimeter and higher, for example), in order to be feasible. An inexpensive and simple cathode which is capable of operating in this environment employs materials such as graphite, carbonized felt, or other substances having the property that when suitably excited there results a high temperature plasma of positive and negative charges which can serve briefly as a source of electrons and therefore be considered to be a plasma cathode. Enhancing the operating cycle time of such a plasma cathode is a principal concern of the present invention.
The generation of a surface plasma in this manner occurs through enhancement of the electric field intensity occurring at a large number of small projections or irregularities found on the surface of the cathode material. Such electric field enhancement can accomplish the field emission of electrons. This emission can be forced to high current densities by a suitable applied voltage, a voltage which tends to thermally heat the cathode surface projections to explosively high temperatures and thereby form a plasma of positive and negative charged particles.
The explosive formation of such a plasma is accompanied by imparting a movement velocity normal to the cathode surface to the plasma. This movement velocity typically ranges from about five kilometers per second at an accelerating potential of 150 kilovolts to velocities of 20 to 30 kilometers per second with accelerating potentials of several hundred kilovolts.
This plasma surface motion is in effect a current flowing through an evacuated structure which contains the cathode and anode elements. The nature of this current is defined by the Child-Langmuir equation which may be expressed as: EQU J=2.34E-6 V.sup.3/2 /d.sup.2 ( 1)
where
J=current density (A/sq meter) PA1 V=cathode to anode voltage (volts) PA1 d=cathode to anode spacing (meters)
In using the Child-Langmuir equation the effective cathode to anode spacing, d, is the distance measured between the moving plasma surface and the anode electrode. Since the plasma is in motion, however, this spacing distance d changes with time. As the spacing d decreases, for example, the current density for the same cathode to anode voltage can be observed to increase according to the Child-Langmuir relationship. Such current change as a result of spacing decrease can be counterproductive, and may be accommodated or compensated in a particular electron device by varying the cathode to anode voltage. However, such change influences the operating power level and may also produce other changes in the operating characteristics of the device. For example, if the operating device is a virtual cathode oscillator, a VICATOR, the oscillating frequency is determined by this voltage and to an even greater extent by the cathode to anode spacing and therefore can be predicted to vary with changes in either of these parameters.
In practice, the changing of cathode to anode spacing in a plasma-dependent electronic device is referred to as "closing" of the cathode. Ultimately the plasma cathode reaches the anode and thereby closes the cathode to anode space to a final value of zero. Typically, this closing action also has the undesirable effect of limiting the usable operating time of an electronic device employing the plasma cathode to times in the 100 to 150 nanoseconds range. The overcoming of this operating time limitation is a principle feature of the present invention.
The patent art shows examples of electron apparatus of this general nature. Included in this patent art is the U.S. Patent of D. M. Goebel et al, U.S. Pat. No. 4,297,615, which is concerned with a high current density thermionic cathode structure wherein heat is used to obtain a high density plasma from a lanthanum hexaboride cathode structure. The Goebel et al lanthanum hexaboride cathode element is mounted in a plasma generating chamber and is coupled to a lower plasma density utilization chamber through one or more openings or apertures which are suitably restricted, in order to maintain the plasma density in the cathode chamber above a critical level for obtaining the desired current density from the lanthanum hexaboride material. The present invention is distinguished by the filamentary heating, the lanthanum hexaboride, the high plasma density cathode chamber and the supplying of a gas stream in order to move the plasma in the Goebel et al apparatus.
Also included in this art is the patent of Bernhard Hillenbrand et al, U.S. Pat. No. 4,634,935, and the patent of Wilhelm Huber, U.S. Pat. No. 4,659,963. Both the Hillenbrand et al and Huber patents are concerned with gas discharge display devices wherein a vacuum-tight enclosure is divided into separate spaces and wherein charged particle plasmas are used as a source of electrons for operating the display. The present invention is distinguished from the low operating power levels and other features in the Hillenbrand and Huber patents.
This art also includes the patent of George Wakalopulos et al, U.S. Pat. No. 4,749,911, which is concerned with an ion plasma electron gun housing dose rate control that is achieved by way of amplitude modulation of a plasma discharge. In the Wakalopulos invention, positive ions generated by a wire in a plasma discharge chamber are accelerated through an extraction grid into a second chamber containing a high voltage cold cathode. These positive ions bombard a surface of the cathode, causing the cathode to emit secondary electrons which form an electron beam. The present invention is distinguished over the Wakalopulos electron gun by structural and functional differences which include the secondary electron emission mechanism.